Archive for January, 2008

ANN: HStringTemplate 0.2

HStringTemplate is a general purpose templating system, geared especially towards HTML and based on Terrence Parr’s Java library.

On Hackage at:

Development version at: darcs get

Haddocks at:

Additional documentation on the grammar at: as well as in some posts at

Lots of cleanup, lots of additions, hopefully this is an extremely usable release. Still no group or interface files, but I suspect that those are mainly useful for code-generation, which seems like something Haskell programmers would want to use something other than a templating system for in any case.

On to the good stuff:

  • Now on hackage!
  • Generics. Not one but two types. One set of simple bindings for the standard Data class, and one for syb-with-class. (Alex Drummond’s RJson library, which is really cool, was very helpful in figuring out how to do this).
  • A withContext method that turns any set of name-value bindings into the context for a StringTemplate. Along with the syb-with-class bindings, this should make for relatively seamless interoperability with HAppS.
  • Lots of other additional bindings for working with standard time formats, numeric types, etc.
  • Encoders. A standard mechanism for HTML-escaping strings (or javascript-escaping, or urlencoding them, or etc.) that A) ensures it happens uniformly and B) ensures it happens no more than once.
  • Improved pretty printing support, eliminating corner-cases where wrapping did not occur.
  • Creation of directory groups is now done properly in the IO monad, with caching functionality moved to the scarily-named unsafeVolatileDirectoryGroup.
  • 80% Top-Level testing coverage in the base. (And more to come, but I only have so much time).

And of course, patches and feedback always welcome.

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Turtles all the way.

HStringTemplate had a very irritating hack in it up until about twenty minutes ago. Every time we worked with some of StringTemplate’s fancier constructs, particularly chaining template application (i.e. $foo:bar():baz():etc()$ which applies foo to bar, feeds it to baz, then feeds it in turn to etc) or using a template as an attribute (i.e. ${$foo$}$ which creates an anonymous template, that in turn grabs foo from the outer scope) the value had to be rendered down to a string before passing it back into the next scope. Now, this normally would have been unfortunately ugly, but not that harmful. However, because of the wrapping and indentation support that HStringTemplate provides, it meant that we couldn’t get proper indentation in such a case since we didn’t know where to wrap the expression yet (i.e. how far it would be indented) at the time we evaluated it.

That last ugliness is now gone. The approach is genuinely turtles all the way down, as the saying goes. This not only makes things work right, but it makes the entire data model cleaner and more efficient. And, while there were a number of changes to type signatures to pass the right information around (sometimes I feel type signatures are the Dorian Gray’s portrait of the Haskell world — the more elegant your code gets, the more complicated and unwieldy your signatures), the end result required only three changes to lines of actual code, and actually resulted in something slightly smaller!

I hope to get to this in a later blog post, but the lesson is that static typing and type inference aren’t just about bondage and discipline and safety. They can also give you a sort of incredible expressive power, where the information on what you’re doing is carried around invisibly in the type system as a sort of secret parameter that can give you a great deal of control over the way your code actually behaves. While the Java version of StringTemplate, for example, uses separate renderers that control how output displays, everything in HStringTemplate (lexing, parsing, construction of functions, establishing pretty-printing relationships) can be done in a single pass. And thanks to laziness and higher-order functions, this pass can produce partially applied functions where everything else (such as indentation levels) can be figured out in a second single pass at runtime. (NB: this idiom, completely natural to Haskell, is sometimes known as runtime compilation although it’s a different use of the term than is usually the case.)

So what does that mean to the end user? Well, you can do this, for one:

> putStr $ PP.renderStyle ( {PP.lineLength=80}) $ toPPDoc $
  setAttribute "foo" [[(1::Double)..30],[200..230]] $ newSTMP
  "Some long text: $foo:{Some label: [$it$]}:{#$it$#};separator=', '$ END"

Some long text: #Some label: [1.0, 2.0, 3.0, 4.0, 
                              5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 
                              14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0, 21.0, 
                              22.0, 23.0, 24.0, 25.0, 26.0, 27.0, 28.0, 29.0, 
                 Some label: [200.0, 201.0, 202.0, 203.0, 204.0, 
                              205.0, 206.0, 207.0, 208.0, 209.0, 210.0, 211.0, 
                              212.0, 213.0, 214.0, 215.0, 216.0, 217.0, 218.0, 
                              219.0, 220.0, 221.0, 222.0, 223.0, 224.0, 225.0, 
                              226.0, 227.0, 228.0, 229.0, 230.0]# END

So what’s happening with this expression? The newSTMP call, as usual, parses a string into a template. The setAttribute call inserts, “foo”, a list of two lists of doubles, into the template. The toPPDoc call evaluates the template in the context of the attributes we’ve set, and returns a Pretty Printer Doc. The render call is to the Text.PrettyPrint.HughesPJ library (which I’ve imported, qualified as PP), and tells it how long we want the lines to be, and finally, the putStr call displays the returned string with newline characters appropriately rendered.

Now onto the mechanics of the StringTemplate itself. First we print some text, and then foo, as a list, is iterated over and repeatedly passed into the next part of the expression. The sublists of foo are each printed, preceded by “Some label: ” and surrounded in braces. The values of each sublist are separated by commas, because we set that option in the top level expression, and that gets propagated downwards as well, and then the whole resulting expression gets wrapped in pound signs by the final template application. And the new cool part is, because everything is “turtles all the way,” those sublists are not rendered and wrapped until the entire shebang is called, meaning that not only do we preserve multiple levels of indentation, but that when those sublists are displayed, everything, including the pound signs that were only inserted *after* we evaluated the sublists of foo, is taken into account such that they indent and wrap properly.

The beauty of programming in a language that supports abstractions as elegantly as Haskell does is that doing it this way not only gives nicer performance, and nicer features, but that it creates cleaner, simpler code. Implementing this feature properly didn’t mean fighting the language, but just giving into to what the types had been whispering to me all along.

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More Simple HStringTemplate Examples

Today I pushed a few changes to the repo so that it builds properly with GHC 6.8. Beyond that, I realized, with a little prompting, that I foolishly hadn’t exported a decent function to query groups of StringTemplates. Additionally, ac on #haskell (the friendliest programming IRC channel this side of the millennium) asked for a simple function that generalized setAttribute to set a whole bunch of attributes at once. The following simple example shows the use of groups, the setManyAttrib function, and how to globally insert options for a single or multiple templates to boot.

> let foo = newSTMP "foo" :: StringTemplate String
> let bar = newSTMP "bar $foo()$" :: StringTemplate String
> toString foo
> toString bar
"bar No Template Found for: foo"
> let grp = groupStringTemplates [("foo",foo),("bar",bar)]
> toString <$> getStringTemplate "foo" grp
Just "foo"
> toString <$> getStringTemplate "fbar" grp
> toString <$> getStringTemplate "bar" grp
Just "bar foo"
> let baz = optInsertTmpl [("separator","; ")] $ newSTMP "$a$ $b$ $c$" :: StringTemplate String
> toString $ setManyAttrib [("a","first"),("b","second"),("c","third")] $ baz
"first second third"
> toString $ setAttribute "a" ([1..4]::[Int]) $ baz
"1; 2; 3; 4 "
> let grp2 = groupStringTemplates [("foo",foo),("bar",bar),("baz",baz)]
> fmap toString $ getStringTemplate "baz" . optInsertGroup [("null","nothing")] $ grp2
Just "nothing nothing nothing"

I still haven’t even touched all the nice bits of the StringTemplate grammar, particularly its recursive constructs. But that’s enough for now.

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HStringTemplate: An Elegant, Functional, Nifty Templating Engine for Haskell.

HStringTemplate is a port of Terrence Parr’s lovely StringTemplate engine to Haskell.

It is available, cabalized, at:
darcs get

As interest has grown in using Haskell for web applications, there has been an increasing buzz about the need for a good templating engine in Haskell. Why might we need this? After all, Haskell has lovely combinator libraries for generating HTML programmatically, enforcing good form through its type system. But sometimes, we don’t want well-formed HTML. We want the ugly stuff that floats around to deal with eight varieties of browser incompatibilities and the latest silly ajax trick. Or sometimes we’re working with a team of graphic designers, and they want to work in almost-HTML. Or sometimes we just want to be able to change the design of a web application on the fly, completely independent of our program logic, and of, heavens forbid, recompiling and possibly messing with a live application.

So template engines are popular, and indeed, considered a key part of most web frameworks out there. One problem — they’re mainly awful, imperatively-conceived behemoths that capriciously mix program logic with display and, consequently, entail a great deal of overhead. Enter StringTemplate, a nifty and fairly-well developed template format that’s both pure and functional, and therefore pretty much the only one of its kind. Indeed, it also seems to be getting heavy use in code generation because its paradigm maps neatly to traversing parse-trees.

HStringTemplate is not feature-complete, and indeed is only at version 0.1. But it should implement pretty much everything you’ll find here, only nicer, because it’s in Haskell. There are scads of different recursive constructs and ways to handle inclusion and inheritance. Furthermore, HStringTemplate handles conditionals, and also a very Haskellish implementation of custom rendering.

Templates can be constructed that return strings, ShowSs, bytestrings, or even pretty printer Docs that handle wrapping, indentation, and fill elegantly. Even better, these templates are parsed and compiled only once, after which point there isn’t a syntax tree anymore, just a function that operates on the environment of attributes that have been passed to it.

Ok. Enough talk. Let’s look at a sample GHCi session. Note that when defining “hello” we have to give the signature explicitly, as it’s not clear until later what type of StringTemplate we’re expecting.

> let hello = newSTMP "something $foo;separator='; '$ something" :: StringTemplate String
> toString $ setAttribute "foo" "SomethingElse" hello
"something SomethingElse something"
> toString $ setAttribute "foo" ["a","b","c"] hello
"something a; b; c something"
> toString $ setAttribute "foo" ([1..10]::[Double]) hello
"something 1.0; 2.0; 3.0; 4.0; 5.0; 6.0; 7.0; 8.0; 9.0; 10.0 something"

And that should be enough to get you started! Be sure to check the haddocks as well, which cover the API in some more detail.

Where I take it from here depends in part on what sort of response I get, so patches, gripes, API comments and feature requests are all more than welcome.

Meanwhile, I’ve learned quite a bit doing this, and suspect that once the benchmarks are in, this will in fact turn out to be a fairly lean and mean implementation compared to the Java one. As a matter of fact, it currently stands at, excluding tests, 414 lines of code. The Java version, admittedly somewhat more featureful, runs in the vicinity of 12,000! Over the next few weeks, I plan to blog about some of my “aha” moments, mainly involving monoids and their many delicious instances.

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